Motor drive systems using speed controls are in common usage and are successfully applied in many applications. A typical motor drive system uses an AC/DC converter connected via a DC bus to a DC/AC converter. The DC bus includes a DC bus filter capacitor. The motor drive system is controlled by a speed controller. Even though this configuration has proven reliable and durable, it suffers from the ripple current limitation of the DC bus filter capacitors. In the very act of performing its purpose as a filter, the capacitors conduct the AC portion of the rectified bus current directly across the converter. The ripple current flowing through the DC bus filter capacitors' internal resistance creates heat and raises the internal temperature of the capacitors reducing their life expectancy. Excessive ripple current can cause premature and catastrophic failure of the capacitors.
In many rural areas, due to unavailability of three-phase AC power, three-phase Variable Frequency Drives (VFDs) are often required to operate from single-phase AC source. To overcome this issue, many VFD manufacturers simply de-rate the drive. Since the RMS value of the input current when using a single-phase AC source is at least two times higher than with a three-phase AC source, there are certain issues that have to be addressed when using three-phase drives with single-phase AC source. Important issues while operating VFDs from single-phase AC source are input rectifier diode average and peak currents, capacitor heating due to higher ripple current, exceeding the current rating of input terminal blocks, etc. Depending on a particular drive, either one of the three factors mentioned above can be the weakest link. Hence the de-rate method needs to consider all three concurrently rather than just the DC bus voltage ripple as is presently practiced. Some of the traditional methods being used at present are discussed below.
Most of the concerns that arise when a single-phase AC supply is used to power up a three-phase motor drive system are generally addressed by severely de-rating the VFDs. The de-rating technique relies on the ripple current capacity of the DC bus capacitor in the VFD. Most electrolytic capacitor manufacturers provide 100 Hz or 120 Hz ripple handling capacity at a given temperature around the capacitor can. Using this information, most drive manufacturers calculate a single-phase power rating for the drive. This theoretical calculation is often verified by experimental tests to confirm that the core temperature of the capacitor does not exceed its rated value. A look up table for a variety of drive sizes is typically provided which the end users can refer to while de-rating the three-phase VFDs to operate from single-phase AC source.
In some cases where the VFD has a built-in DC link choke, the actual ripple current into the DC bus capacitor is naturally reduced and allows the VFD to handle a slightly higher load when powered from a single-phase AC source. Such options are made available by most drive manufacturers. Some drive manufacturers also suggest adding an input inductor of 0.01 pu˜0.03 pu to reduce the capacitor ripple current to some extent.
In the frequency foldback method which is based on sensing the ripple voltage across the DC bus capacitors, as described in U.S. Pat. No. 7,330,779, the basic premise is that the DC bus ripple current amplitude can be predicted by measuring the ripple voltage across the DC bus capacitor. Since DC bus capacitor current cannot be directly measured, the ′779 patent suggests using the DC bus capacitor voltage ripple, instead. The ripple across the DC bus voltage is measured and the output frequency of the drive is reduced accordingly to limit the output power and in the process limit the ripple current through the capacitor. Though this method in its various forms has been adopted in the industry, as shown in U.S. Pat. No. 6,244,825, and seems to limit the power rating of three-phase VFDs operating from single-phase AC source, it does not account for the stresses in other parts of the VFD, especially the rectifier diodes and the input terminals.
Traditional implementation of this method involves a simple proportional-integral (PI) control algorithm that has a ripple value set point based on the three-phase rating of the VFD and a simple look up table. The DC bus voltage is scanned periodically and the maximum and minimum values of the DC bus voltage during that period are used to calculate the ripple voltage. This value is used as the feedback in the ripple regulator PI loop. The set point is stored in a lookup table since it is predetermined based on the maximum allowable capacitor ripple current typically at 100 Hz˜120 Hz and 60° C. for a given capacitor. If the measured DC bus ripple voltage feedback value exceeds the regulation level, the foldback function is triggered and the output speed is decreased. Decreasing the output frequency decreases the load on the VFD and limits the capacitor ripple current. It should be pointed out here that the capacitor ripple current is not directly measured. The DC bus capacitor voltage ripple is monitored and the capacitor ripple current is assumed to be directly proportional to it.
Though the frequency foldback based on DC bus voltage ripple method is simple and easy to use, certain issues have been observed in the field. The most important of all is that the DC bus capacitor ripple voltage tolerance level may be higher than the peak to peak current rippling handling capability of input diode rectifiers or the RMS current capacity of the input terminal blocks, especially in smaller sized VFDs. Hence, reducing the output frequency to only satisfy the ripple current rating of the DC bus capacitor may not be sufficient to limit the stresses in other components of the VFD.
Moreover, the bus ripple voltage foldback method is not a direct control method. The DC bus voltage ripple may not reflect the true state of the capacitor. If the capacitor has deteriorated in capacitance, the average voltage across it can be quite low while the ripple could still be within specified limits. A lower DC bus voltage will cause higher input current for a given load level and can adversely affect the input diodes and terminal blocks. Similar sequence of events can happen if the single-phase AC source feeding the drive is a weak AC source. The available DC bus voltage, due to a weak AC source can again be lower than normal and affect the output power being delivered by the drive. Hence, relying solely on the DC bus voltage ripple is not sufficient as it neglects external factors including input voltage, source impedance, effective capacitance, etc. that can affect the load level at which fold-back occurs, leading to poor utilization of the drive.
In some cases, even a three-phase AC source behaves like a quasi single-phase AC source as an imbalance in the system increases. This is typical of weak AC sources. Under light load condition, the AC source may appear to be fine but when the load increases, the weak AC system starts to behave abnormally and show imbalanced voltages such that the ripple in the DC bus and the peak diode current increase.
The present application is directed to improvements in operating a three-phase variable frequency drive from an unbalanced three-phase or single-phase AC source.